A great deal of attention has been directed towards the identification and characterization of factors capable of stimulating the growth and proliferation of specific cell types. In the last twenty-five years, a number of such mitogenic factors have been isolated. Rather than having highly specific activities as may have been originally anticipated, many such growth factors are now recognized to have multifunctional activities, affecting a wide spectrum of cell types. In addition, certain activities are shared by homologous members of a family of growth factors.
One family of growth factors now known to have a broad spectrum of activities is the fibroblast growth factors (FGF). Basic FGF is a protein which has a molecular weight of approximately 16 kD, is acid and temperature sensitive and has a high isoelectric point. A structurally related protein, acidic FGF, has an acidic isoelectric point. FGFs exhibit a mitogenic effect on a wide variety of mesenchymal, endocrine and neural cells. Of particular interest is their stimulatory effect on collateral vascularization and angiogenesis. Such mitogenic effects have stimulated considerable interest in FGF as potential therapeutic agents for wound healing, nerve regeneration and cartilage repair, for example.
Cells that respond to basic FGF have been shown to possess specific receptors on the cell surface membranes. The receptor proteins appear to be single chain polypeptides with molecular weights ranging from 110 to 150 kD, depending on cell type. The proteins bind basic FGF with high affinity (Kd=10-80 pM), with receptor numbers ranging from 2000 to 80,000 per cell. The receptors can be purified from rat brain, using a combination of lectin and ligand affinity chromatography and are associated with tyrosine kinase activity (Imamura et al., Biochem. Biophys. Res. Comm., 155:583-590 (1988); Huang and Huang, J. Biol. Chem., 261:9568-9571 (1986), both of which are incorporated herein by reference).
On baby hamster kidney cells (BHK), two basic FGF receptors with estimated molecular weights of 110 and 130 kD have been reported (Neufeld and Gospodarowicz, J. Biol. Chem., 260;13860-13868 (1985); Neufeld and Gospodarowicz, J. Biol. Chem., 261:5631-5637 (1986), both of which are incorporated herein by reference). Both receptor proteins bind basic FGF and acidic FGF, although it appears that the larger binds basic FGF preferentially while the smaller has somewhat higher affinity for acidic FGF.
In addition to potentially useful proliferative effects, basic FGF-induced mitogenic stimulation may, in some instances, be detrimental. For example, cell proliferation and angiogenesis are an integral aspect of tumor growth. Basic FGF is thought to play a pathophysiological role, for example, in tumor development, rheumatoid arthritis, proliferative diabetic retinopathies and other complications of diabetes.
There thus exists a need for being able to inhibit certain mitogenic effects of basic FGF within the body which may give rise to pathological conditions; however, such inhibition must be accomplished in a way that does not result in the death of the animal or the infliction of substantial harm thereto. Because of the ubiquitous distribution of FGF target cells and presumably FGF receptors throughout the body, it was felt that such an objective could not be accomplished, yet the present invention satisfies this need.